Sand production in oil and gas wells is a serious problem mainly due to sand induced erosion. It is therefore of great interest to accurately detect the presence of sand and the amount of produced sand to maximise the oil/gas production rate and still maintain sand-free production.
Sand can be measured either with intrusive sensors, i.e. obstructions in the oil/gas flow, or with non-intrusive sensors. Intrusive sensors can either be based on measuring the erosion of the obstruction/probe, or on measuring the acoustic emission generated when the particles hit the obstruction. Conventional non-intrusive sensors are based on ultrasonic (PZT) transducers mounted at bends in the pipe, where particles will impact the inside of the pipe wall, generating an ultrasonic pulse which is picked up by the acoustic sensor. Non-intrusive sensors are much preferred unless intrusive sensors can offer significantly better performance. However, non-intrusive sensors will require bends, and are believed to be less sensitive than intrusive sensors.
Acoustic sensors should be able to measure acoustic signals at frequencies >100 kHz, or ideally >500 kHz, where sand noise is dominant over other noise sources, to provide unambiguous sand monitoring with high signal-to-noise ratio. Other noise sources include flow generated noise, mechanical/structural noise and noise from electrical equipment (eg. from electrical submersible pumps). Provided the individual hits can be separated in time, the quantity of produced sand can be derived from the number of hits and the signal amplitudes resulting from each hit. To be able to reliably detect sand particles and verify sand-free production with acoustic sensors, extreme sensitivity with large signal-to-noise ratio is required. Sand particles of interest have diameters ranging from 50–400 micrometers.
Fibre optic interferometric sensors are known to offer high sensitivity and resolution for dynamic measurands, which make them particularly attractive for acoustic sensing, eg. as hydrophones, see for example [T. G. Giallorenzi et.al., “Optical fiber sensor technology,” IEEE J. Quantum Electron., Vol. 18, pp. 626–665, 1982]). The small dimensions of an optical fibre provide the potential for high frequency acoustic sensing, and the use of fibre optic interferometric sensors for ultrasonic acoustic sensing has been investigated [N. Lagaros et.al. “Ultrasonic acoustic sensing,” Proc. SPIE, Vol. 798, pp. 94–101, 1987], [D. Wiesler et.al., “Fiber optic ultrasound sensors for medical imaging applications”, 12th Intern. Conf. on Optical Fiber Sensors, Willamsburg, USA, pp. 358–361, 1997.]. A fibre optic interferometric sensor typically consists of two optical paths, where the optical path length difference is modulated by the measurand. The interferometer is normally excited by a laser source and the changes in differential optical path length causes a modulation of the light intensity at the output of the interferometer. It is known that the sensitivity and resolution is improved by using a high coherence laser source.
One known high coherence laser source is the fibre distributed feedback (DFB) laser [U.S. Pat. No. 5,771,251 to J. T. Kringlebotn et.al.], which consists of a single fibre Bragg grating providing feedback in a gain fibre, typically an erbium-doped fibre pumped by a semiconductor laser. Such a laser typically has a coherence length of several kilometers. It is further known that such a laser also can be used as a sensor element [U.S. Pat. No. 5,844,927 to J. T. Kringlebotn], for example for acoustic sensing, where the acoustic field modulates the stresses in the fibre laser and hence the optical frequency of the fibre laser, which can be measured using an optical interferometer which converts the frequency fluctuation into intensity fluctuations. The low coherence length of the laser allows the use of large path length imbalance in the interferometer and hence a high sensitivity. It is known that several fibre DFB lasers can be wavelength multiplexed along one optical fibre. Finally, it is also known that several interferometric sensors can be multiplexed along one or several optical fibres, for example by using Fabry-Perot type interferometers based on pairs of low-reflectivity FBG reflectors, where each pair has a different Bragg wavelength.
Fibre optic sensors are passive, with no electrical parts/wiring, and can provide reliable operation at high temperatures up to at least 200° C. The large bandwidth of an optical fibre also means that an almost unlimited amount of high frequency raw data can be transmitted along the fibre.
Interferometric techniques combined with high coherent sources allow highly sensitive dynamic measurements with low noise, hence providing good signal-to-noise ratio measurements. The potentially small dimension of these fibre optic sensors, in particular the DFB fibre laser sensor, allows for high frequency acoustic sensing [D. Thingbø, E. Rønnekleiv, and J. T. Kringlebotn, “Intrinsic distributed feedback fibre laser high-frequency hydrophone,” Techn. Dig., Conf. on Bragg gratings, Photosensitivity, and Poling in Glass Waveguides,” pp. 57–59, Florida, US, Sep. 23–25, 1999].